1. Field of the Invention
This invention relates generally to methods for obtaining subsurface geological information in the exploration of petroleum deposits. More particularly, the invention relates to a method for determining geometry of subsurface features while drilling.
2. Discussion of the Related Art
Much of the ultimate success (or failure) of hydrocarbon recovery depends on the assessment, usually implicit, of the internal crack-, pore-, and stress-geometry, and pore fluids of the reservoir rock. It also depends on the estimation of the changes to these quantities as the secondary and tertiary production or enhanced oil recovery (EOR) proceeds. At present, only a small proportion of the oil in a reservoir is recoverable. Numerous techniques have been developed to enhance the recovery of petroleum deposits, almost all of which depend on subsurface information.
Historically, the geological structure of a subsurface region has been determined by arranging seismic detectors on the surface above the region to be explored. A seismic source, also located on or near the surface, is actuated to produce one or more seismic pulses: each pulse propagates downward as an expanding spherical wave front and reflected by acoustic impedance changes in the subsurface. The acoustic impedance changes generally coincide with changes in lithology, structure or both. The reflected wave front arrives at the surface where it is detected by the seismic sensors. The sensors generate signals which are recorded for later processing, display, and interpretation. One disadvantage in the technique is that the seismic signal is altered (filtered and attenuated) by each formation it traverses before being detected. Thus the signal reflects characteristics of all of the formations traversed.
A technique generally known as Vertical Seismic Profiling (VSP) is also used to obtain subsurface information. In VSP, minimum length travel paths of the seismic wave front are detected. In order to conduct a VSP survey a string of seismic detectors are lowered into a bore hole with the seismic source located at the surface. The source is actuated to produce one or more seismic pulses. Each pulse propagates downward as an expanding spherical wave front and is detected by the sensors disposed within the bore hole. Once the signals are detected, the sensors may be moved to another position and the procedure repeated. Alternatively, the source may be located in the bore hole and the sensors disposed along the surface in the desired array. A major disadvantage with VSP is that a bore hole is required. If one is not present, the bore hole must be drilled and the drill string removed before the survey can be conducted. After the survey, the drill string may be reinserted in the bore hole to continue drilling. The "tripping" of the drill string in and out of the bore hole in order to conduct the survey requires a great deal of time and is thus costly.
Another technique very similar to VSP is cross-well tomography. The technique requires two well bores, one for the seismic source and one for the seismic sensors. Both the source and the sensors are positioned at predetermined level and the source is actuated. The seismic signal propagates through the subsurface and is detected by the sensors in the adjacent well. This technique provides critical information on the seismic velocities for the various subsurface intervals. However, this technique suffers the same disadvantages as traditional VSP surveys. It is most effective in regions where the wells have already been drilled.
Other techniques employ down hole seismic sources. Such sources typically make use of an artificial transducer situated at a given depth in the bore hole. Broding et al., U.S. Pat. No. 3,909,776, and U.S. Pat. No. 3,881,168 issued to Farr and Ward use a fluid driven oscillator to generate seismic waves in the earth from a position in the bore hole. Phase delays between a geophone located on the surface near the well and another located near the top of the well are used to produce a log of travel-time and compressional wave velocity as a function of depth. Broding et al. use a fluid driven oscillator which changes emitted frequency as a function of time, much like a swept frequency source such as described in U.S. Pat. No. 2,688,124.
One technique for obtaining subsurface information utilizes the drill bit as a seismic source. One of the earliest patents concerning down hole sources is Weatherby, U.S. Pat. No. 2,062,151, which discloses using the drill bit as an impulse generator. Drilling is done with a cable tool, which is dropped on the hole bottom, thus creating the seismic impulses. The bit location and wave velocity can be obtained from these impulses. Drill-bit-generated direct wave arrival time differences between two geophone locations are used to determine rock acoustic velocity. U.S. Pat Nos. 4,363,112 and 4,365,322 issued to Widrow disclose using the continuous natural, random vibrations of a rotary drill bit to launch seismic waves into the earth. Spectral amplitudes and interference patterns are used to image subsurface reflectors. Advantages in these techniques are that they may be used in the drilling of "wildcat wells", wells that are drilled in frontier regions with unknown geology. However, no art in the field of detecting-while-drilling teaches using shear waves generated by the drill bit to detect fracture zones in a subsurface zone of interest. Moreover, the art in the field of detecting-while-drilling does not teach nor suggest using shear wave energy while drilling horizontally.
It is a general object of this invention to provide a method for obtaining information about subsurface intervals of interest while drilling. It is another object of this invention to provide a method for detecting fracture zones in a formation penetrated by a drill bit. It is yet another object of this invention to provide a method for real-time detection of fracture zones in formations while drilling substantially horizontally or between adjacent stratigraphic units.